Fluid inclusions in upper mantle xenoliths from Northern Patagonia, Argentina : evidence for an upper mantle diapir

VARELA, M; BJERG, E A; CLOCCHIATTI, R; KURAT, G; LABUDIA, C; BJERG, E A

Mineralogy and Petrology

Springer

Lugar: Viena; Año: 1997 vol. 60 p. 145 - 164

Three generations of fluid inclusions can be recognized in upper mantle xenoliths fromalkali basalts of the Somoncura Massif, Northern Patagonia, Argentina. The first("early", "primary") one consists of dense CO2 inclusions which were trapped in themantle-crust boundary zone (22-36 km minimum trapping depth). Their co-geneticrelationship with silicate melt inclusions enables us to constrain their minimumtrapping temperature at 1200°C, indicating a high temperature event in a coolerenvironment. The "late" ("pseudosecondary" and "secondary") generations of fluidinclusions were classified in accordance with their homogenization temperature toliquid CO2 (L1) and vapor CO2 (L2) phase. The minimum trapping depth for the first ofthe late inclusions (L1) is about 16 kin. In spite of the uncertainties related to this value,L1 inclusions indicate that the upper mantle rocks, of which samples were delivered bythe basalts, had some residence time in the middle crust where they experienced ametasomatic event. The fact that this event did not destroy the earlier inclusions, placessevere constraints on its duration. The second late inclusions (L2) are low-pressure COainclusions with a minimum trapping depth of only 2 kin, presumably a shallow magmachamber of the host basalts. The succession of fluid inclusions strongly points toward afairly fast uprising upper mantle underneath Northern Patagonia. The petrology andmineral chemistry of the peridotitic xenoliths support this view. Extensive partial melting and loss of these melts is indicated by the preponderance of harzburgites in theupper mantle underneath Northern Patagonia, a fairly unusual feature for a continentalupper mantle. That depletion event as well as several metasomatic events - includingthose which left traces of fluid inclusions - are possibly related to a high-speed diapiricuprise of the upper mantle in this area. The path can be traced from the garnet peridotitestability field into the middle crust, a journey which must have been unusually fast.Differences in rock, mineral, and fluid inclusion properties between geographiclocations suggest a diffuse and differential type of diapirism. Future studies willhopefully help to map the full extent and the highs and lows of this diapir and elucidatequestions related to its origin and future.